a) Field of the Invention
The present invention relates to a solid state image pickup device having photoelectric conversion elements and its control method.
b) Description of the Related Art
FIG. 7 is a plan view of a general CCD type solid state image pickup device. The solid image pickup device is formed on a semiconductor substrate 1 made of silicon or the like and having a two-dimensional flat plane. The semiconductor substrate 1 has a plurality of photodiodes (photoelectric conversion elements) 2, vertical charge transfer paths (VCCD) 3, transfer gates (read gates) 4, a horizontal charge transfer path (HCCD) 6, and an output amplifier 7.
The photodiodes 2 are disposed in a two-dimensional matrix shape. Each photodiode converts received light into electric charges and stores the charges. Each vertical charge transfer path 3 is formed on the left side of each column of photodiodes 2 via transfer gates 4. The transfer gate 4 reads electric charges in the corresponding photodiode 2 to the vertical charge transfer path 3.
The vertical charge transfer path 3 is made of charge coupled devices (CCDs) and transfers electric charges read from the photodiodes 2 from the upper area of the drawing of FIG. 7 to the lower area (in a vertical direction). The horizontal charge transfer path 6 is made of CCDs, receives electric charges of one row unit transferred from the vertical charge transfer paths 3 and transfers the charges from the left area of the drawing of FIG. 7 to the right area (in a horizontal direction).
The output amplifier 7 outputs a voltage corresponding to the amount of electric charges transferred from the horizontal charge transfer path 6. This voltage value corresponds to a pixel value. Each photodiode 2 corresponds to a pixel. By disposing the photodiodes 2 two-dimensionally, signals of a two-dimensional image can be obtained.
The number of pixels of a solid state image pickup device of a recent digital camera is increasing greatly in order to improve the image quality. Digital cameras having a solid state image pickup device having a number of pixels, one million pixels or larger generally called mega pixels, are being developed. As the number of pixels of a solid state image pickup device increases, a time taken to read image signals of one frame prolongs essentially. Although a prolonged read time of image signals does not become a fatal problem in reproducing a still image, a moving image is associated with a problem that the image signal read time cannot follow the frame rate (generally {fraction (1/30)} sec for NTSC signals).
As the number of pixels exceeds one million, it is difficult to read moving images at the frame rate of {fraction (1/30)} sec. Therefore, a solid state image pickup device of a mega pixel order does not reproduce moving images obtained by all pixels on a display, but reduces the data amount (pixel number) to reproduce moving images so as to follow the frame rate.